Chemical Vapor Deposition (CVD) is the process of depositing a solid material from a gaseous phase onto a substrate by means of a chemical reaction. The deposition reaction involved is generally thermal decomposition, chemical oxidation, or chemical reduction. In one example of thermal decomposition, organometallic compounds are transported to the substrate surface as a vapor and are reduced to the elemental metal state on the substrate surface.
For chemical reduction, the reducing agent most usually employed is hydrogen, although metal vapors can also be used. The substrate can also act as a reductant as in the case of tungsten hexafluoride reduction by silicon. The substrate can also supply one element of a compound or alloy deposit. The CVD process can be used to deposit many elements and alloys as well as compounds including oxides, nitrides and carbides.
Since their invention integrated circuits [ICs] have been fabricated from silicon, oxygen and aluminum in elemental or compound form. More recently, a new class of refractory materials is being added to basic elements that have been sufficient for so long in response to changing materials requirements dictated by smaller IC feature size, higher current densities and the desire for better electrical performance. Design requirements have narrowed the choice of materials to those which exhibit low contact resistance to silicon, high resistance to the effects of electromigration at contacts or steps, and to materials which can be deposited at temperatures low enough to preserve ever shallower junctions.
Refractory metals can be deposited in various forms as refractory metal silicides, as blankets of refractory metals, or selectively on other metal surfaces without deposition on adjacent dielectric such as silicon dioxide or silicon nitrate. The selective deposition saves masking in etch steps and provides self-aligning structures.
Tungsten and other refractory metals are being seriously considered for use in advanced ICs. CVD selective tungsten has been shown to meet a variety of requirements on VLSI and ULSI chips such as a diffusion barrier, etch barrier, via fill, low resistance shunt of source/drain regions and on gates in pure or silicide form. Tungsten is also being applied as an IC fabrication aid, such as self-aligning scratch-resistance pads for in-process testing, stencils for etch selectivity enhancement and masks for x-ray lithography, low reflectivity coatings, etc, and even for novel passive micro devices such as squids, bridge wires, etc.
In addition to its favorable physical properties, tungsten is of particular interest because it can be deposited selectively on silicon, metals, or silicides, so that it is truly self-aligning. Tungsten can also serve a volume filling function, thus enhancing planarity, a high priority in multi-level chip designs. Because it can be deposited without additional masks, process complexity is reduced with a concommitment cost-saving. Increasing use of in-process testing is expected to enhance yields. As IC technology is driven to small feature size and multiple interconnect levels, the selective deposition capability of a CVD process will become ever more important.
CVD of refractory metals offer some advantages over sputtering. CVD refractory metals can provide good step coverage, reduce system complexity, and yield higher purity deposits. To take advantage of these benefits a CVD process with high throughput is required. Early CVD refractory metal work for VLSI was done in cold-wall reactors operating at atmospheric pressures with small batch sizes. Reduced pressure, cold-wall, CVD technology offers the possibility of producing refractory films for VLSI in large quantities.
In the present invention, CVD technology can be used to manufacture deposits on substrates for a variety of purposes. Tungsten carbide and aluminum oxide wear coatings on cutting tools; corrosion resistant coatings of tantalum, boron nitride, silicon carbide and the like and tungsten coatings on steel to reduce erosion can be applied according to this invention. The apparatus and method is particularly advantageous in manufacturing solid state electronic devices and energy conversion devices.
Chemical vapor deposition of electronic materials is described by T. L. Chu et al, J. Bac. Sci. Technol. 10, 1 (1973) and B. E. Watts, Thin Solid Films 18, 1 (1973). They describe the formation and doping of epitaxial films of such materials as silicon, germanium and GaAs, for example. In the field of energy conversion, the CVD process provides materials for nuclear fission product retention, solar energy collection, and superconduction. A summary of the chemical vapor deposition field is provided by W. A. Bryant, "The Fundamentals of Chemical Vapour Deposition" in Journal of Materials Science 12, 1285 (1977).
The deposition parameters of temperature, pressure, the ratio of reactant gases, and amount and distriution of gas flow critically determine the deposition rates and the ability of a particular system to provide the desired uniformity and quality of deposition. The limitations of prior art systems stem from their inability to adequately control one or more of these factors from deposit contamination.
The reaction chambers employed for chemical vapor deposition are generally classified as cold-wall or as hot-wall systems. In cold-wall systems, the substrate is heated by inductive coupling, radiant heating or direct electrical resistance heating of internal support elements. Hot-wall systems rely on radiant heating elements arranged to create a heated reaction and deposition zone. Conduction and convection heating approaches have also been used in hot-wall system.
Cold-wall systems for chemical vapor deposition are described in U.S. Pat. Nos. 3,594,227, 3,699,298 and 3,916,822. In these systems, the semiconductor wafers are positioned inside a vacuum chamber, and induction coils are arranged exterior to the vacuum chamber. The wafers are mounted on a susceptible material adapted for heating by RF energy. By localizing heat to the immediate semiconductor wafer area, chemical vapor deposition is limited to the heated areas. Since the unheated walls are below CVD temperatures, deposition on the walls is reduced. The temperatures in the reaction zone are usually not as uniform as those obtained with hot-wall systems, and it is impossible to control the temperature across individual wafers.